1. Field of the Invention
This invention relates to a rotational directional solidification crystal growth system and method, and in particular, to a system and method using Coriolis and centrifugal forces caused by the rotation of a crucible or the system to grow crystals according to the Bridgman Method so as to reduce the nature convection caused by gravity, which will improve the axial and radial dopant distribution.
2. Description of the Related Art
In the various techniques of crystal growth, materials for growing a single crystal include semiconductors, organics, inorganics (oxides), metals, superconductors etc. Currently, the major methods for growing such crystals include the Czochralski method, the floating-zone method, the Bridgman method, and the gradient freeze method, which is similar to the Bridgman method.
Generally speaking, the crystal yield with the Czochralski method is higher than that of the Gradient Freeze Method, although the Czochralski method may produce more defects due to thermal stress. Therefore, such crystals, except silicon single crystals, are usually made ether by the Bridgman method or by the gradient freeze method, while floating-zone method is less suitable for large size crystals.
In the Bridgman method, a crucible is moving in a furnace from a high-temperature zone to a low-temperature zone so as to change the temperature of the crucible. In contrast, in the gradient freeze method, the temperature of crucible is decreasing without moving the crucible. In both the Bridgman method and the gradient freeze method, the temperature of the environment is stable and single crystals can be grown steadily. Therefore, both methods can provide optimal crystal growth conditions to manufacture high quality and low defect single crystals. However, the growth of single crystals is always accompanied by solidification heat, so that, as shown in FIG. 1, interface 13, between solid crystal 11 and melt 12, is formed with a concave center. In this case, melt 12 can be any one of the materials mentioned above. Furthermore, the natural convection can concentrate dopant 14, which distributes in melt 12, at the central of interface 13. In other words, the nature convection will cause the axial and radial segregation of dopant 14 and will cause supercooling and breakdown in interface 13. As shown in FIG. 1, arrow G indicates the direction of gravity, and arrow C indicates the direction of natural convection. As described above, even though heat transfer is controlled accurately during crystal growth, the convection of melt 12 cannot be eliminated completely. Therefore, the crystals may have a poor dopant distribution, as shown in FIG. 3A. Thus, it is very important to control the convection so as to reduce the axial and radial segregation of the dopant so that the axial and radial distribution of dopant can be controlled efficiently.
In most conventional methods, for decreasing the effects of natural convection, an additional magnetic field is used to reduce the partial accumulation of dopant during crystal growth. However, the crystal growth system with a magnetic field is not only hard to be implemented and expensive, but it also hard to provide the magnetism efficiently around the crystal growth area to control the growth. Additionally, the system with a magnetic field can grow crystals only when the melt is electrically conductive.
Referring to FIG. 2, currently a centrifugal force is used to reduce partial convection so as to improve the axial segregation. In this case, a large-scale centrifuge 21 is used to rotate the crucible 22, wherein the crucible 22, usually, is freely rotated. Therefore, the direction of resultant acceleration due to the centrifugal and the gravity forces is parallel with the axis of the crucible 22. However, the method mentioned above doesn""t utilize the centrifugal force and the Coriolis force very well, so that the convection of melt 23 is a three-dimensional flow and the radial segregation of the dopant 24 is increased.
Therefore, it is an important issue under study to provide a system and method for reducing convection, and eliminating the segregation due to the central concave and breakdown interface, so as to decrease the axial and radial segregation and prevent the overcooling and breakdown of the interface.
An objective of the invention is to provide a rotational directional solidification crystal growth system and method for reducing the natural convection caused by gravity, so as to improve the distribution of the dopant and increase the quality of the crystals.
Another objective of the invention is to provide a system and method for eliminating concave center of the interface caused by the accumulation of dopant or solute, and enhancing the stability of interface.
To achieve the above objective, the rotational directional solidification crystal growth system according to the invention includes a furnace, a crucible, and a rotation support device. In this invention, the vertical furnace provides a high-temperature condition and a low-temperature condition. The crucible has a seed well and a growth region. The seed well contains a seed crystal, and the growth region contains a raw material and a dopant distributed in the raw material. The temperature of crucible can be changed depending on the high-temperature and low-temperature conditions so as to solidify the raw material and grow a directionally solidified single crystal. The rotation support device supports and rotates the crucible, wherein the tangent velocity of the rotated crucible is no less than 5xcfx80/3 cm/s (centimeters per second).
This invention also provides a rotational directional solidification crystal growth method. The method includes the steps of providing a crucible, heating the crucible, rotating the crucible, and cooling the crucible. In this case, the tangent velocity of the rotated crucible is no less than 5xcfx80/3 cm/s, and the raw material is solidified and the crystal is grown initiating from the seed crystal.
It is important that those who skilled in the art may rotate the crucible with low speed in order to increase uniformity of crucible heating. In other words, in conventional, the objective of rotation is to increase heating uniformity of crucible. Thus, the conventional objective is obviously different from that of the invention, and the tangent velocity of the rotated crucible is far smaller than what is claimed (5xcfx80/3 cm/s) in this invention.
According to the invention, the rotational directional solidification crystal growth system and method rotate the raw material and dopant in a tangent velocity of no less than 5xcfx80/3 cm/s, so that centrifugal force and Coriolis force are sufficiently supplied to the raw material and dopant. Therefore, the central concave of interface caused by the dopant accumulation can be reduced, the stability of interface can be enhanced, and the nature convection can be counteracted so as to improve the distribution of dopant (both in axial and radial) and increase the quality of crystals.